The present disclosure relates generally to immersion lithography processes used for the manufacture of semiconductor devices, and more particularly to the immersion lithography systems' capability for the control and containment of the immersion lens liquid during the systems' processing operations.
The manufacture of very large-scale integrated (VLSI) circuits requires the use of many photolithography process steps to define and create specific circuits and components onto the semiconductor wafer (substrate) surface. Conventional photolithography systems comprise of several basic subsystems, a light source, optical transmission elements, photo mask reticles, and electronic controllers. These systems are used to project a specific circuit image, defined by the mask reticles, onto a semiconductor wafer coated with a light sensitive film (photoresist) coating. As VLSI technology advances to higher performance, circuits become geometrically smaller and denser, requiring lithography equipment with lower resolution (smaller feature size) projection and printing capability. Such equipment is now required to be capable of producing features with feature resolutions smaller than 100 nanometers (nm). As new device generations are developed requiring even further improvements, of feature resolutions 65 nm and lower, major advancements to photolithography processing are required.
Immersion lithography has been implemented to take advantage of the process technology's capability for much improved resolution. Immersion lens lithography features the usage of a liquid medium to fill the entire gap between the last objective lens element of the light projection system and the semiconductor wafer (substrate) surface during the light exposure operations of the photoresist pattern printing process. The liquid medium used as the immersion lens provides an improved index of refraction for the exposing light, thus improving the resolution capability of the lithographic system. This is represented by the Rayleigh Resolution formula, R=k1λ/N.A., where R (feature size resolution) is dependant upon k1 (certain process constants), λ (wavelength of the transmitted light) and the N.A. (Numerical Aperture of the light projection system). It is noted that N.A. is also a function of the index of refraction where N.A.=n sinθ. Variable n is the index of refraction of the liquid medium between the objective lens and the wafer substrate, and is θ the acceptance angle of the lens for a transmitted light.
It can be seen that as the index of refraction (n) becomes higher for a fixed acceptance angle, the numerical aperture (N.A.) of the projection system becomes larger thus providing a lower R value, i.e. a higher resolution. Conventional immersion lithographic systems utilize de-ionized water as the immersion fluid between an objective lens and the wafer substrate. At one of the wavelengths, for example 193 nm, de-ionized water at 20 degree Celsius has an index of refraction at approximately 1.44 versus air which has an index of refraction at approximately 1.00. It can be seen that immersion lithographic systems utilizing de-ionized water as the immersion fluid, offers a significant improvement to the resolution of the photolithography processes.
FIG. 1 is a cross-sectional diagram that illustrates a typical immersion lithography system. The immersion printing section 100 of the lithography system contains a movable wafer chuck/stage 102 incorporated with vacuum channels 104 for holding and fixing the photoresist coated wafer 106 onto the top of the wafer chuck/stage 102. The immersion fluid 108 is shown located on top of the photoresist coated wafer 106 displacing the entire volume of space between the wafer and the last objective lens element 110 of the lithography's light projection system. The immersion fluid 108 is in direct contact with both the top surface of the photoresist coated wafer 106 and the lower surface of the objective lens element 110.
There are two fluid reservoirs directly connected to the fluid of the water immersion area 109. A fluid supply reservoir 112 serves as the means for supplying and injecting the immersion fluid into the immersion area 109 just under the objective lens element 110. The injected immersion fluid is either held by capillary forces immersion area or contained within a fixture moving with the lens. A typical thickness of the immersion fluid is between 1 to 2 millimeters (mm). A fluid recovery reservoir 114 serves as the means for recovering and accepting the output fluid flow from the immersion lens 108. It is noted that the immersion fluid flow is of the direction starting from the fluid supply reservoir 112, through the immersion area 108, and out to the fluid recovery reservoir 114. There may be associated mechanical hardware and electrical/electronic controllers by which the flow of immersion fluid as described above, is managed and controlled. The large downward arrows 116 of FIG. 1 located above the lithography system's last objective lens element 110 represents the direction of and the transmission of the pattern image-exposing light 116 towards the objective lens element 110 and through the immersion lens 108 to the photoresist coated wafer 106. During normal operation of the immersion lithography printing of the photoresist coated wafer 106, the wafer chuck 102 moves to position each exposure target area of the wafer under the fixed locations of the immersion fluid 108, the fluid reservoirs 112 and 114, the objective lens element 110 and the pattern image-exposing light 116.
The typical immersion lithography system as configured and described by FIG. 1 is effective for performing the immersion lithography process operations. There are several issues with the practical aspects of the physical configurations and procedures that may influence the quality of the immersion lithography process as well as the operational efficiency of the system. FIG. 2 helps to illustrate these issues. FIG. 2 is a cross-sectional diagram of a typical immersion lithography system, similar to FIG. 1, but showing the hardware component locations during processing at the edge of the wafer substrate. The immersion printing section 200 of the lithography system is shown with the movable wafer chuck/stage 202 incorporated with vacuum channels 204 for holding and fixing the photoresist coated wafer 206 onto the top of the wafer stage 202. The immersion fluid 208 is shown located on top of the photoresist coated wafer 206 displacing the entire volume of space between the wafer and the last objective lens element 210 of the lithography's light projection system. The fluid 208 is in direct contact with both the top surface of the photoresist coated wafer 206 and the lower surface of the objective lens element 210. The two fluid reservoirs, the fluid supply reservoir 212 and the fluid recovery reservoir 214, are directly connected to the fluid 208.
The immersion fluid 208 is shown located at the edge of the wafer substrate 206 to perform processing upon the photoresist areas at the wafer edge. At the wafer substrate edge, the normally closed-loop flow of the immersion fluid from the fluid supply reservoir 212, through the immersion area 209 to the fluid recovery reservoir 214 is different than that previously described for FIG. 1. As shown in FIG. 2, there is now an additional path 215 for the output flow of the immersion fluid which occurs when processing at the wafer substrate edge. This additional path 215 allows the immersion fluid to flow out of the immersion lens 208, reservoirs loop along the outside edge of the wafer substrate 206, and the outside edge of the movable wafer chuck/stage 202, not returning back into the fluid recovery reservoir 214. This uncontrolled, non-containment of immersion fluid itself may not necessarily affect the quality of the immersion lithography process, but may adversely affect the system's operational efficiency. The immersion fluid is not fully recovered, wasted as it flows away from the immersion lens 208 and immersion fluid reservoirs. In addition, the mechanical and electrical components of the wafer chuck 202 and other underlying assemblies may be undesirably wet from the additional flow 215 of immersion fluid. Such undesired wetting and flow may contaminate and shorten the operational life of the system's hardware and electrical components. This issue may require immersion lithography system designers to invest additional time and costs consuming efforts to accommodate the system design and configurations for this additional flow of immersion fluid.
The wafer edge position of the immersion lens 208 also exposes the immersion lithography processing to certain quality issues. During normal wafer processing within the semiconductor processing facilities, the wafer edges have a high propensity to collect particulate contamination. This is due to the fact that the wafer edge is handled more and in closer proximity to particulate generation sources than the interior areas of the wafer substrate. As shown by FIG. 2, when the wafer chuck/stage 202 positions the wafer substrate edge 206 under the immersion lens 208, the immersion fluid contacts particulates located at the wafer substrate edge 206. As result, the particulates may become dislodged from the wafer substrate surface 206 and becomes suspended within the fluid 208. These particulates may then affect the immersion lithography exposing processes enough to distort and disturb the printed image patterns on the wafer substrate. They may also deposit and adhere onto the wafer substrate surface to affect subsequent wafer processing operations. The flow of the immersion fluid to the recovery reservoir 214 and the additional flow 215 out of the immersion area 208 may not be able to keep the particulates from affecting the immersion lithography and subsequent processing operations.
What is desired is an improved system for the sealing and control of the immersion fluid within the immersion area throughout the entire immersion lithography process operations. The improved system would also minimize the introduction of particulates into the immersion fluid by preventing the immersion fluid from contact with the particulate contamination areas. The system would help maintain the integrity of the photoresist image and pattern on the wafers such that they do not become distorted and defective.